Trapping-Mediated Surface Reactions

Even nondissociative (molecular) adsorption may be accompanied by an activation barrier if, for example, the reaction proceeds from a physisorbed state into a chemisorbed state (trapping-mediated adsorption). In the system 02/Pt(l 11), for example, the O2 molecule may be chemisorbed either in a superoxo-like or a peroxo-like state [18]. It was found that with low kinetic energies of the incident molecules, both types of surface species are formed, while at higher kinetic energies the more strongly held peroxo-like species is favored, thus reflecting correlations between incident translational energy and preferred trajectories for adsorption [19]. 

These dendrimers show one oxidation, due to Ru(III)/Ru(II) couple, and two reduction processes, attributed to the first and second reduction of the ligands. Scanning the potential to more negative values, adsorption onto Pt electrode occurs and charge trapping peaks are observed. These peaks likely arise from redox centers that are electronically isolated from the electrode surface, so that their redox reactions are mediated by adjacent redox sites. Morphological changes of the deposited film have been observed upon application of a potential difference and they have been attributed to the deposition or dissolution of the dendrimer and/or to ejection or adsorption of counterions and/or solvent into the film. 

The modelling of kinetics at modified electrodes has received much attention over the last 10 years [1-11], mainly due to the interest in the potential uses of chemically modified electrodes in analytical applications. The first treatment published by Andrieux et al. [5] was closely followed by a complimentary treatment by Albery and Hillman [1, 2]. Both deal with the simplest basic case, that is, the coupled effects of diffusion and reaction for a second-order reaction between a species freely diffusing in the bulk solution and a redox mediator species trapped within the film at the modified electrode surface. The results obtained by the two treatments are essentially identical, although the two approaches are slightly different. 

In this connection it is useful to specify that the capture cross section, which describes the reactivity of the species in the solution towards holes photoproduced in the semiconductor, may have not the same physical meaning for various reactions. In fact, because of the highly exothermic nature of the hole transfer from the valence band of Ti02 (placed at ca. 3 V versus RHE) to redox couples stable in aqueous solution, such processes are generally considered to be mediated by the surface states located into the band gap. In particular, hydroxyl groups on the titania surface have frequently been mentioned as possible hole traps(cf. reaction (24)). 

The existence of Arnold diffusion is irrelevant to the properties of separatrix manifolds, which still mediate the transport of chaotic trajectories within the regions of phase space they control. However, if Arnold diffusion is present in a given multidimensional system, the possibility exists for chaotic motion initially trapped between two nonreactive (trapped) KAM layers to eventually become reactive. This would presumably manifest itself as an apparent bottleneck to the rate of population decay, as chaotic trajectories slowly leak out from the region occupied by regular KAM surfaces into the portion of phase space more directly accessible to the hypercylinders. However, transport via the Arnold diffusion mechanism typically manifests itself on time scales much larger than those that we observe in numerical simulations (Arnold diffusion usually occurs on the order of thousands of mappings, or vibrational periods), and so it seems improbable that this effect would be observed in a typical reaction dynamics simulation. It would be interesting to characterize the effect of Arnold diffusion in realistic molecular models. 

Figure 1.7 The generalization of laser chemistry. Top panel laser-mediated gas-phase reaction middle panel laser-mediated reaction of a molecule trapped in a (cluster) solvent cage bottom panel laser-mediated reaction of an adsorbed molecule with a surface atom/molecule (laser interacts with adsorbed molecule or the surface)...

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